If SqlDatareader fetches one record at a time, and not one field at a time, then lazy load pattern wasn’t…

Question

Hello,

Does SqlDataReader fetch one record at a time from DB or one field at a time?

Assume the following query returns a single row:

select columns_1, column_2, column_3
from   some_Table

and suppose readerS ( readerS is an instance returned by SqlCommand.ExecuteReader() )only reads *column_3* before closing the connection:

readerS[“column_3”].ToString();

Now will readerS fetch from sql server just a *“column_3”* value, or will it fetch all 3 columns, even though it will only read one?

Reason I’m asking this is because I’m currently reading a book on website programming and author mentions that he will code DLL objects to use lazy load pattern.

In short - when DLL object is created, a DB query is performed (via DAL), which retrieves data from various columns and with it populates the properties of our DLL object. Since one of the fields ( call it "L" ) may contain quite a substantial amount of text, author decided to retrieve that field only when that property is read for the first time.

But when I looked at stored procedure used by author to retrieve columns and populate DLL object's properties ( except for property L, which won't get populated ), I've noticed that procedure also retrieved L column, even though it won't be used to populate a corresponding property. Now if DataReader retrieves a whole row, and not just the field we actually read, then author didn't really implemented lazy load pattern?!

Answer 1

I believe the data reader only works on the row level, not field by field.

Without seeing the code my guess would be that the lazy loading comes in to play when populating the properties of the class, not the pulling of data from the database.

The lazy loading would be that the class property is not populated with the data from the database until something exiplicitly tries to access the property. In this case "L" is not loaded untill the "L" property's get method is called.

Answer 2

I don't know how the author implemented the pattern, but DataReader retrieves result row by row not column by column.